PHP is what is called a "server-side scripting language". It is what allows websites to interact with users in a dynamic manner. Without scripting, there would be no such thing as Facebook, MySpace, Google, Amazon, or really any other modern website.
So here's to PHP! And here are a few links to some sites where you can learn more about it, because quite frankly, there's more information out there than I can hope to cram into a blog entry...